Keven:
I lived in the DC area for 20 years. Although I wasn't a pilot then, I know the area well and have often planned to fly up there in my Skymaster.
Let me offer two suggestions.
Manassas Regional (
http://www.airnav.com/airport/KHEF ) is on the SW outskirts of the DC Metropolitan Area. Although it's inside the new ADIZ which surrounds DC, which requires that you be in contact with ATC and have a discrete squawk code (either IFR or VFR Flight Following), it's in a fairly uncongested area and easy to get to (especially coming in from the South like you will), although you need a rental car to get downtown because a taxi would be expensive.
Another choice is Potomac Airfield (
http://potomac-airfield.com/ ) belonging to David Wartofsky (301 248-5720) who owns a Skymaster and used to be a frequent (although ultra-conservative) contributor to Peter's website. This is probably the airport closest to downtown DC, and therefore inside the ADIZ, which requires that you be in contact with ATC and have a discrete squawk code (either IFR or VFR Flight Following). But 100LL is $1.79 for airfield-based aircraft and maybe David will extend the same courtesy to you.
In terms of advertised fuel prices, I use
www.airnav.com and it shows that College Park (KCGS) is the cheapest around at $2.00, but the downside is that it's on the North side of all the congestion, so it's not as easy to get to.
Ernie